OBJECTIVE: The long range objective is to utilize isolated nucleoli of the Novikoff ascites tumor and other rat tissues for study of the synthesis and processing of ribosomal RNA and ribosomes in vitro. The immediate objective is to pursue the characterization of the nucleolar ribosomal RNA methylases and their role in the regulation of ribosome production in mammalian cells. APPROACH: Nucleoli were prepared from tumor cells, tumor cells pretreated with anti-tumor agents, liver, or liver pretreated with agents to stimulate liver nucleolar activity. These nucleolar preparations were then incubated in vitro for the characterization of rRNA methylases in response to stimulatory and inhibitory effectors. Nucleolar RNAs from these various sources were also labeled in vivo with RNA precursors, and the labeled nucleoli were incubated in vitro to examine the stability of nucleolar RNA in correlation to the activity of rRNA methylases.